Among optical recording media that have come into focus as large-capacity and high-density memories is an erasable optical recording medium that allows rewriting of information. Currently, the development of the erasable optical recording medium has been underway. In one form of the erasable optical recording medium, information is recorded and erased utilizing thermal energy generated by irradiation with a laser beam. The recording medium includes a transparent substrate of a general disk shape and a recording layer provided on the substrate. The recording layer is formed of a thin film in which a phase change is caused between an amorphous state and a crystalline state.
Phase change materials known to be used for the recording layer include an alloy film mainly containing elements such as Ge, Sb, Te, and In of, for example, a GeSbTe alloy. In many cases, information is recorded in such a manner that the recording layer is partially brought into an amorphous state to form a mark and erased in such a manner that the mark in the amorphous state is brought into a crystalline state. When heated to a temperature equal to or higher than the melting point and subsequently cooled at a speed higher than a fixed speed, the recording layer is brought into the amorphous state. When heated to a temperature equal to or higher than the crystallization temperature and equal to or lower than the melting point, the recording layer is brought into the crystalline state.
Generally, on the substrate, guide groves (grooves) in the form of a spiral or concentric circles for tracking a laser beam in recording and reproducing information and addresses for indicating a position on the recording medium, each composed of uneven strings of pits, are provided to form an initial state of the substrate. A region between the adjoining grooves is referred to as a land. In many cases, information is recorded on one of the groove and the land, and the other serves as a guard band for separating adjoining recording tracks from each other.
In recent years, the improvement in processing capabilities of various kinds of information-processing equipment has allowed the processing of an increasing amount of information. Thus, a recording medium has been requested to allow larger-capacity information recording and reproducing. In order to attain this, DVD-RAM or the like has employed a method in which information is recorded on both of the groove and the land, so that a higher track density can be obtained. In this case, the groove and the land are set so as to be substantially equal in width. Recording media of this kind have employed a method in which address information is recorded in an intermediate position between a pair of adjoining groove and land tracks so that with respect to the pair of adjoining groove and land tracks, one address in formation is recorded.
An address recorded in this manner in the intermediate position between the pair of adjoining groove and land tracks is referred to as “an intermediate address”. Further, a method in which the intermediate address is used to record address information so that the address information is shared by a pair of adjoining tracks is referred to as “an intermediate address method”.
In JP10(1998)-31822 A, a method of demodulating address information in a recording medium employing the intermediate address method is disclosed. In the method, a sum signal or a difference signal of electric signals output from a photodetector provided in an optical head of an optical disk device is used to demodulate the address information. The photodetector includes light receiving parts divided into two parts in a direction parallel to tracks on the recording medium.
In this connection, a signal quality assessment was conducted using reproduction signals obtained by reproducing an address on a recording medium formed in the following manner. As shown in FIG. 10, address pits 9 were arranged in the form of staggered pit strings so that with respect to a distance (a track pitch) Tp between a center line of a groove track 7 and a center line of a land track 8, center lines of strings of the address pits 9 were shifted in a radial direction of the recording medium (namely, a direction perpendicular to the tracks) at a distance of about Tp/2 from the center lines of the groove tracks 7 and the center lines of the land tracks 8. In the recording medium, a pit width W of the address pits 9 was the same as the track pitch Tp (namely, the same as the width of the groove track 7 and the width of the land track 8). As a result, the reproduction signals obtained by reproducing the address differed in symmetry between the sum signal and the difference signal. For each of the sum signal and the difference signal, an optimum condition under which excellent signal quality could be obtained was found by adjusting the lengths of the address pits. However, no condition was found under which such signal quality could be obtained for both of the sum signal and the difference signal at the same time.
That is, neither of the following cases allows sufficient signal quality to be obtained, which has led to a problem of a limited margin for reproduction conditions. In one case, a recording medium suited for address information demodulation using the sum signal is employed in an optical disk device in which address information demodulation is performed using the difference signal. In the other case, a recording medium suited for address information demodulation using the difference signal is employed in an optical disk device in which address information demodulation is performed using the sum signal. In other words, in each of the optical disk devices in which address information demodulation is performed using the sum signal and the difference signal, respectively, a permissible level of variations in address forming conditions of recording media is limited.